Plasmid pGS146 carries the Escherichia coli gcv system on a 7.12 kb SalI-BamHI DNA insert fragment. The DNA sequence of a gene which presumably encodes the T-protein of the glycine cleavage (GCV) enzyme complex was determined. The gene, designated gcvT, encodes a polypeptide of 364 amino acids with a calculated molecular weight of 40,146 daltons. In a minicell system, the SalI-BamHI fragment directs the synthesis of three polypeptides with Mr values of about 93,300, 43,300 and 17,400 daltons. When gcvT was inactivated by insertion of a translation terminator sequence, the Mr 43,300 dalton polypeptide was not observed. The deduced amino acid sequence of the E. coli T-protein was compared with the sequence of the T-protein from bovine liver. 190 of 364 amino acid residues are identical or chemically similar between the two proteins. An S1 nuclease mapping experiment located the transcription start point for gcvT. Single basepair changes were made in the promoter -10 and -35 sequences. These mutations significantly reduced expression from a gcvT-lacZ gene fusion. The gcvT gene is transcribed and translated in the same direction as the gcvH gene.